An example of the vehicle of this kind is disclosed in Japanese Patent Laid-Open No. 08-295140. According to the teachings of Japanese Patent Laid-Open No. 08-295140, the first gear element of the differential gear unit is coupled to the generator, the second gear element is coupled to the motor to serves as the output element, and the third gear element is coupled to the breaking means. The third gear element is also coupled to the engine via the clutch. Thus, the engine is allowed to be disconnected from the third gear element by disengaging the clutch. Therefore, when driving the vehicle by the power outputted from the motor and the engine, the engine is allowed not only to be rotated but also stopped.
In turn, Japanese Patent Laid-Open No. 2010-144851 also discloses a hybrid vehicle in which the engine is connected with the motor-generator through the first clutch, and the motor-generator is connected with the automatic transmission through the second clutch. According to the teachings of Japanese Patent Laid-Open No. 2010-144851, the torque transmitting capacity of the second clutch is controlled by allowing the second clutch to slip thereby achieving a demanded driving torque when the vehicle is stopping or running at a low speed. In addition, if the thermal load on the slipping second clutch is large under the situation that the vehicle is driven by the torques of the engine and the motor-generator by engaging the first clutch, slip amount of the second clutch is reduced to suppress temperature rise of the second clutch.
A driving mode of the vehicle can be diversified into a plurality of different modes by thus interposing the clutch between the engine and the motor as taught by Japanese Patent Laid-Opens Nos. 08-295140 and 2010-144851. However, if a friction surface of the clutch is partially moisturized, friction coefficient of the moisture portion becomes lower than that of the remaining portion. As a result, friction coefficient of the friction surface becomes uneven, and clutch judder may be caused if the clutch is engaged. In this situation, a clutch torque may be varied to generate a torque pulse or shocks thereby worsening noise and vibrations (i.e., NVH characteristics). In turn, if the thermal load on the clutch is increased excessively as a result of slipping the clutch, durability of the clutch may be deteriorated and the friction coefficient is reduced. As a result, the clutch judder may be caused.